Identifying key molecules that modulate drug experience-induced brain reward plasticity remains an important goal of current drug abuse research. We recently identified a novel regulatory mechanism by which cocaine stimulates the transient dephosphorylation of HDAC5 at S279, which induces nuclear accumulation of this chromatin-modifying enzyme. We also find that dephosphorylation of this site is critical for HDAC5 to limit cocaine reward and to reduce cocaine-primed reinstatement of drug seeking. In addition, we find that the closely-related enzyme, HDAC4, is oppositely regulated by chronic cocaine exposure, suggesting an important role for this regulation in the development of cocaine-induced behavioral plasticity. In this grant we seek to characterize the differential regulation of HDAC4 and HDAC5 and to test the importance of this regulation for cocaine reward and drug relapse behaviors in vivo. To this end, we propose the following: Specific Aim 1: In this aim, we will characterize the differential phosphorylation of HDAC4 and HDAC5 in response to cocaine exposure, in order to better understand the potential role of this regulation for cocaine reward and drug seeking behaviors. We will use established conditions and novel reagents generated in our lab to assess phosphorylation and subcellular distribution of these HDACs. Specific Aim 2: In this aim, we will utilize viral-mediated gene delivery to express phosphorylation site mutants of HDAC4 in the mouse NAc. In addition, we will use novel floxed HDAC4 conditional KO mice and viral-mediated cre expression in the adult NAc to test the necessity of HDAC4 for limiting cocaine-induced behavioral adaptations. Specific Aim 3: In this aim, we will extend our findings that cocaine triggers transient nuclear accumulation of HDAC5 to limit cocaine reward by testing the effects of the dephosphorylated HDAC5 in the most relevant model for cocaine addiction, intravenous self-administration of cocaine. Our initial findings reveal a suppression of cocaine-prime reinstatement when the dephosphorylated form of HDAC5 is expressed. We seek to expand upon these studies to determine the effect of nuclear HDAC5 on cocaine taking and seeking, motivation to work for cocaine, and propensity to reinstate drug seeking behaviors.